Purification of lower aliphatic acids



originally contained therein. .acids are merely heated to an elevated temperature without having present therein a salt, the final product will PURIFICATION OF LOWER ALIPHATIC ACIDS Adin L. Stautzenberger and Alexander F. MacLean, Cor- N Drawing. Application March 12,- 1956 1 Serial No. 570,688

9 Claims. Cl. 260-540) This invention relates to lower aliphatic acids and trite Sttes Patent 0 relates more particularly to a process for the purifica-' of lower aliphatic hydrocarbons such as propane and].

butane, they are obtained in admixture with a large number of other compounds including, for example, acetaldehyde, biacetyl, acetone, methyl ethyl ketone and butyrolactone. To recover the lower aliphatic acids from the oxidation reaction mixture, such mixture is subjected to a complex series of distillations. While it is possible in this way to obtain lower aliphatic acids of relatively high purity, such-acids are frequently deficient with respect to their permanganate time and sulfuric acid color owing to the presence therein of small-proportions of residual impurities. -Since the permanganate time and sulfuric acid color are important commercial tests which the acids must meet for many uses, the presence therein of such impurities is highly objectionable. The removal from the lower aliphatic acids of these impurities by conventional distillation techniques is not commercially feasible.

It is an important object of this invention to provide a process for the purification of lower aliphatic acids containing from 2 to 4 carbon atoms which will produce a product that will meet all commercial specifications.

Other objects of this invention will be apparent from the following detailed description and claims.

According to the present invention, the lower aliphatic acids containing small proportions of residual impurities and which have a short permanganate time and a poor sulfuric acid color are purified by heating the same in the liquid phase to an elevated temperature materially above the boiling point of the acids at atmospheric pressure but below the temperature at which the acids will decompose to any substantial extent. For best results, the heating of the acids is carried out in the presence of a small proportion of a suitable salt and, preferably,

a small proportion of water. Following the heating, the acids are distilled to separate them from the decomposition products which have formed from the impurities When the lower aliphatic have a satisfactory permanganate time, but its sulfuric acid color will still be somewhat poor. When the heating is carried out in the presence of a salt and, preferably, also in the presence of water, the final product will have both a satisfactory permanganate time and also a I good sulfuric acid color so that it will be well suited for wide commercial use.

The lower aliphatic acids that may be treated in accordance with the process of this invention, namely,

acetic acid, propionic acid and butyric acid, have a relatively high purity as evidenced by the fact that they will normally analyze 99.5% by weight of the acid, or

ice

' hydroxycarbonylic compounds and ethylenically unsaturated compounds.

The lower aliphatic acids should be heated in the liquid phase to a temperature of at least 235 C., but below the temperature at which substantial decomposition of the acids will occur. Best results are obtained when the acids are heated to between 235 and 330 C. To maintain the acids in the liquid phase at such temperatures, a high superatmospheric pressure must be applied thereto. This may be the autogenous pressure developed by heating the acids in a closed vessel when the heating operation is carried outon a batch basis. The superatmospheric pressure may also be applied'to the acids by means of a pump or the like when the heatingof the acids is carried out in a continuous manner by forcing the acids into and through a heating chamber. The time during which the acids are. maintained at an elevated temperature will vary somewhat depending on the precise temperature and other conditions during heating, but will normally be at least 1 hour and, preferably, between land 20 hours.

The best results are obtained when there is present in the lower aliphatic acid during the heating period an ionizable salt whichshould be soluble in the acid. Examples of suitable salts for this purpose are the sodium *and potassium salts of the acid being treated. The

quantity of such salts should range between 0.2 and 10% by weight or, preferably, between 2 and 8% by weight, based on the Weight of the acid being treated. It is also desirable to have present in the lower aliphatic acid during the heating period a small proportion of water ranging from 1 to 10% by weight or, preferably, from 2 to 8% by weight based on the weight of the acid.

During the heating period, the impurities present in the lower aliphatic acids decompose and also condense to high boiling compounds, but there is substantially no decomposition or condensation of the acids themselves. A portion of the decomposition products are gaseous and may simply be vented from the acids during or after the heating period. The remainder of the decomposition products and the condensation products are liquid and remain in the acids, imparting thereto a dark color. To recover the purified lower aliphatic acids, the acids containing the decomposition and condensation products are fractionally distilled. The lower aliphatic acids obtained in this manner show both a good permanganate time and a satisfactory sulfuric acid color so they are well suited for commercial use.

The following examples are given to illustrate this invention further.

Example I Acetic acid which has been prepared by the liquid phase oxidation of propane and butane and which has been recovered from the oxidation reaction mixture and purified by fractional distillation so that it analyses 99.5 by weight of acid, about 2000 parts per million of impurities, has a permanganate time of 0 minutes (2 hours acceptable) and a sulfuric acid color of 5% transmission, is entered into an autoclave. The autoclave is sealed and the contents thereof heated to 285 C. and held at this temperature for 16 hours, during which time the pressure in the autoclave reaches approximately 500 pounds per square inch gage. The autoclave is then vented to permit the gaseous decomposition products to escape and the acetic acid, which has a dark color, is

Example 11 The process of Example I is repeated, but there is added to the acetic acid before it is introducted into the autoclave 4% by weight of sodium acetate. When the acetic acid is distilled, a permanganate time of 2 hours is reached after only 8.3% of the acid has distilled over.

Example III The process of Example I is repeated, but there is added to the acetic acid before it is introduced into the autoclave 4% by weight of sodium acetate and 5% by weight of water. After'distilling an initial cut of 5% of the acid, the acid coming over has a permanganate time in excess of 4 hours and, for some cuts, in excess of 8 hours. The sulfuric acid color is 98% transmission.

In all the above examples, a titration shows that, within the limits of titration error, there has been no loss of acetic acid.

It is to be understood that the foregoing detailed description is merely given by way of illustration and that many variations may be made therein without departing from the spirit of our invention.

Having described our invention, what we desire to secure by Letters Patent is:

1. Process for purifying an impure lower aliphatic acid containing from 2 to 4 carbon atoms which comprises heating in the liquid phase at a temperature of at least 235 C. but below the temperature at which substantial decomposition of the acid occurs for from about 1 to 20 hours a mass consisting essentially of said impure acid,

0 to 10% by weight of a member selected from the group consisting of the sodium and potassium salts of said acid and 0 to 10% by weight of water.

2. Process according to claim 1 wherein the acid is acetic acid.

3. Process according to claim 2 wherein the temperature ranges from 235 to 330 C.

4. Process according to claim 2 wherein the impure acid to be heated is prepared by liquid phase oxidation of lower aliphatic hydrocarbons and contains up to 2000 parts per million of impurities that reduce its permanganate number and increase its sulfuric acid color.

5. Process according to claim 4 wherein said mass consists essentially of said acid.

6. Process according to claim 4 wherein said mass consists essentially of said acid and 0.2 to 10% by weight of its sodium salt.

7. Process according to claim 4 wherein said mass consists essentially of said acid and 0.2 to 10% by References Cited in the file of this patent UNITED STATES PATENTS Britton Aug. 9, 1938 Groll et al. Sept. 9, 1941 OTHER REFERENCES Timmermans: Physico Chemical Constants of Pure Organic Compounds, 1950, p. 380. 

1. PROCESS FOR PURIFYING AN IMPURE LOWER ALIPHATIC ACID CONTAINING FROM 2 TO4 CARBON ATOMS WHICH COMPRISES HEATING IN THE LIQUID PHASE AT A TEMPERATURE OF AT LEAST 235*C. BUT BELOW THE TEMPERATURE AT WHICH SUBSTANTIAL DECOMPOSITION OF THE ACID OCCURS FOR FROM AOUT 1 TO 20 HOURS A MASS CONSISTING ESSENTIALLY OF SAID IMPURE ACID, 0 TO 10% BY WEIGHT OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF THE SODIUM AND POTASSIUM SALTS OF SAID ACID AND 0 TO 10% BY WEIGHT OF WATER. 